Windows build of nkf 2.1.5

This release provides a Windows build of nkf (Network Kanji Filter).

Built using MinGW-w64 on WSL2.
The Makefile in this fork is customized for Windows, so make directly produces nkf.exe.

This release includes both x64 and x86 (win32) builds.

Source: https://github.com/tomneko/nkf-windows-build